Conventional thermostatic control devices used in home ovens and other heating devices typically have a controller consisting of a bulb located within the heated, area, a bellows or other operator located outside the heated area, and a capillary connecting the bulb and the operator. The entire system, bulb, capillary, and operator, is filled with a thermally responsive material so that as the bulb is exposed to heat, the material expands and thus moves the bellows or operator. Temperature control within the oven is obtained by linking the operator to a switch or valve so that the source of heat to the oven is turned off when the operator has moved a predetermined amount. The system works very well as long as the bulb, capillary, and bellows or other operator remains pressure-tight, so that an increase in the volume of the thermally responsive material results in movement of the operator. However, frequently a leak will develop in the system with the result that the thermally responsive material is permitted to leak out. When this occurs, an increase in heat in the area of the bulb does not cause a change in the position of the operator. In other instances, such as stuck or welded contacts, the source of heat may not be disconnected even though the operator has moved an amount normally sufficient to disconnect it. In either event, there is no control to interrupt the source of heat and a severe overheat situation may result. In extreme cases, this may cause a fire, or at a minimum, may cause the burning of whatever material is contained within the oven or other heating appliance. Thus, it is an object of this invention to prevent such an overheat situation when such a failure of the primary temperature control device occurs. It is a further object of the invention to provide a device which will positively interrupt the source of heat and thus "fail safe" in the event of a failure of the temperature control device.